Optically transitioning thermal detector structures

ABSTRACT

A thermal absorption structure of a radiation thermal detector element may include an optically transitioning material configured such that optical conductivity of the thermal absorption structure is temperature sensitive and such that the detector element absorbs radiation less efficiently as its temperature increases, thus reducing its ultimate maximum temperature.

RELATED APPLICATIONS

This application is a continuation of pending U.S. patent applicationSer. No. 12/799,628, filed on Apr. 28, 2010 and entitled “OpticallyTransitioning Thermal Detector Structures”, which is incorporated hereinby reference in its entirety for all purposes.

The present application is related in subject matter to concurrentlyfiled patent application Ser. No. 12/799,626, entitled “PIXEL-LEVELOPTICAL ELEMENTS FOR UNCOOLED INFRARED DETECTOR DEVICES” by Schimert etal., and concurrently filed patent application Ser. No. 12/799,629, nowU.S. Pat. No. 8,227,755, entitled “PIXEL-LEVEL OPTICALLY TRANSITIONINGFILTER ELEMENTS FOR DETECTOR DEVICES” by Fagan III et al., which areeach incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

This invention relates generally to radiation detector devices, and moreparticularly to optically transitioning thermal detector devices such asmicrobolometer pixel membrane structures for uncooled infrared detectordevices.

BACKGROUND OF THE INVENTION

Infrared (IR) detectors are often utilized to detect fires, overheatingmachinery, planes, vehicles, people, and any other objects that emitthermal radiation. Infrared detectors are unaffected by ambient lightconditions or particulate matter in the air such as smoke or fog. Thus,infrared detectors have potential use in night vision and when poorvision conditions exist, such as when normal vision is obscured by smokeor fog. IR detectors are also used in non-imaging applications such asradiometers, gas detectors, and other IR sensors.

A variety of uncooled infrared detector types have been developed in thepast. Many include a focal plane array (FPA) that includes a substratewith a plurality of detector elements that each correspond to arespective pixel. The substrate includes integrated circuitry which iselectrically coupled to other components of the detector elements, andwhich is commonly known as a read out integrated circuit (ROIC).

Infrared detectors generally operate by detecting the differences inthermal radiance of various objects in a scene. That difference isconverted into an electrical signal which is then processed.Microbolometers are infrared radiation detector elements that arefabricated on a substrate material using traditional integrated circuitfabrication techniques. Microbolometer detector arrays consist of thin,low thermal mass, thermally isolated, temperature-dependent resistivemembrane pixel structures. The microbolometer membrane pixel structuresare suspended over silicon ROIC wafers by long thermal isolation legs ina resonant absorbing quarter-wave cavity design.

FIG. 1 illustrates a conventional prior art uncooled infrared detectorelement 100 with a microbolometer pixel membrane structure 112 thatincludes thin (1000 A to 4000 A) thermally-electrically active layer ofthermally absorbing membrane material of a resistive material likeamorphous silicon (a-Si); amorphous silicon geranium (a-SiGe); orvanadium oxide together with an infrared absorbing thin metal absorberlayer like Ti, TiAl; TiN; or Ni and supporting silicon nitride, siliconoxide; or silicon oxynitride. The microbolometer pixel membranestructure 112 is suspended approximately 2 microns above siliconsemiconductor substrate 110 by long thermal isolation legs 116 that areelectrically connected to the ROIC of the semiconductor substrate 110 bymetal interconnects 108 (typically TiW or Aluminum) via aluminum inputpads 114. Disposed on the surface of the supporting substrate 110 andROIC is a metal reflector 118 (typically Aluminum) to form a resonantcavity structure to enhance infrared absorptance in the membrane of thesuspended microbolometer pixel structure 112. For infrared applications,pixel size typically ranges from 12 um×12 um up to 100 um×100 um.

Primary factors affecting response time and sensitivity ofmicrobolometers are thermal mass and thermal isolation. Microbolometerresponse time is the time necessary for a detector element to absorbsufficient infrared radiation to alter an electrical property, such asresistance, of the detector element and to dissipate the heat resultingfrom the absorption of the infrared radiation. Microbolometersensitivity is determined by the amount of infrared radiation requiredto cause a sufficient change in an electrical property of themicrobolometer detector element. Microbolometer response time isinversely proportional to both thermal mass and thermal isolation. Thus,as thermal mass increases, response time becomes slower since more timeis needed to sufficiently heat the additional thermal mass in order toobtain a measurable change in an electrical property of themicrobolometer detector element. Therefore, it is generally desirable tomake microbolometer pixels that are low in mass in order to achieve ashort thermal time constant, while at the same time maximizingabsorption of radiation.

As shown in FIG. 1, material has been removed from the membrane materialof microbolometer pixel membrane structure 112 in a square orrectangular grid pattern to reduce microbolometer pixel mass and toshorten thermal time constant while maintaining good radiationabsorption characteristics. In FIG. 1, suspended microbolometer detectorpixel structure 112 includes conductive element portions 106 that areoriented both parallel and orthogonal to the path of current flowbetween electrical contacts 102 and 104. As illustrated, openings in theform of square holes 111 are defined by material removed between theelectrical contacts 106 to reduce the microbolometer pixel mass. Theholes are typically dimensioned to be small compared to the radiationwavelength of interest. Microbolometer pixels fabricated with thisstructure are sometimes called diffractive resonant cavities (DRCs).

In the past, detector material optimization has been employed to limitor enhance absorption within specific bands. Polarizers and spectralfilters have been separately fabricated and then mounted at a relativelylarge distance (i.e., a distance greater than the pixel size) over afocal plane array of uncooled infrared detector elements in a mannerthat causes radiation cross talk problems. Monolithically-fabricatedspectral and polarizer filters have been provided for non-thermal photoninfrared detector cells, such as mercury-cadmium-telluride (HgCdTe) andindium-antimonide (InSb) infrared photon detector cells. Attempts havebeen made to physically align and mount previously andseparately-fabricated microlens arrays at a distance of about 10 micronsor greater over and above the upper surface of thermal and non-thermalinfrared detector focal plane arrays, with individual microlenses of themicrolens array being aligned to individual detector elements of thefocal plane array at a distance of about 10 microns or greater over andabove the upper surface of the focal plane array. Such previously andseparately-fabricated microlens arrays are fabricated separately andapart from the infrared detector focal plane arrays themselves for laterassembly thereto. Other examples of previously employed detectorfiltering techniques include neutral density filters, shutters orfilters activated by remote detectors, lens material optimization tolimit or enhance transmission of specific bands, and spectral filteringstructures built into detector packaging.

In other cases, window glass has been coated with thermochromic vanadiumoxide film that darkens with increased temperature to block infraredradiation in response to higher levels of infrared energy. Certainwelding hoods have also been manufactured having a transparent viewingwindow that includes a ferroelectric material and an integral detectionapparatus that detects light or radiation produced by a welding arc.These welding hoods are designed to respond to the presence of adetected welding arc by applying an electric field to the ferroelectricmaterial of the viewing window to cause darkening of the viewing windowin a manner that protects the eyes of the welding operator wearing thehood. Past attempts to provide solar immunity to detector elements haveincluded altering materials of the detector element to broaden its rangeof temperature use, reducing optical speed, and mechanically closing ashutter to block radiation. Most past attempts to provide solar immunityto cameras have involved incorporation of mechanical devices thatincrease system complexity and cost, and which may prevent the camerafrom viewing the scene.

SUMMARY OF THE INVENTION

Disclosed herein are apparatus and methods in which a thermal absorptionstructure of a thermal detector element may include an opticallytransitioning material configured, for example, such that the thermalabsorption capability of the detector element varies with temperature ofthe detector element. Examples of such thermal detector elements includeany radiation detector element that utilizes a thermal absorptionstructure to sense radiation falling incident thereon by measuring atleast one property having a value that changes with temperature.Examples include, but are not limited to, thermal detector elements thatsense radiation by measuring changes in one or more properties ofelectrical resistance, electrical capacitance, electrical voltage,electrical current, electromotive force, etc. Specific examples of suchthermal detector elements include, but are not limited to, thermocoupledetectors, ferroelectric detectors, microbolometer detectors, etc.

A thermal absorption structure of a thermal detector element may includea filter layer composed of one or more optically transitioning materialshaving optical properties that change with temperature, e.g., thatchange from optically transmissive to optically reflective or fromoptically reflective to optically transmissive with increasingtemperature. Examples of such optically transitioning materials include,but are not limited to, thermochromic or phase shifting/phasetransitioning materials that act to selectively transmit radiationdepending on temperature, for example, a semi-transparent material thatis thermochromic such that optical transmission of the semi-transparentmaterial is temperature sensitive. For example, in one embodiment anoptically transitioning material may undergo asemiconducting-to-metallic phase transition with increasing temperature,e.g., from a transparent or at least partially transparent semiconductorstate to a reflective or at least partially reflective metallic state.Particular examples of optically transitioning materials include, butare not limited to, thermochromic or phase transitioning compositions ofgermanium-antimony-tellurium (GST or Ge_(X)Sb_(Y)Te_(Z)), vanadium oxide(VO, VO₂, V_(n)O_(2n-1) such as V₂O₃), tungsten-doped vanadium oxide(W:VO_(X) such as W:VO₂, and stoichiometric variants), niobium oxide,tantalum oxide, Ti₂O₃, Fe₃O₄, Mo₉O₂₆, etc. Further, as describedelsewhere herein, composition of such materials may be varied (e.g., bydoping with other materials such as tungsten, aluminum and/or manganese)to tailor the optical transition (e.g., phase transition)characteristics of the material as a function of temperature.

For example, in one exemplary embodiment a microbolometer pixel membranestructure of a thermal detector element (e.g., for detecting infraredradiation, millimeter wave radiation, etc.) may include an opticallytransitioning material, for example, a semi-transparent material that isthermochromic (e.g., provided as a thermochromic layer of the membraneitself) such that optical transmission of the semi-transparent materialis temperature sensitive. Such an optically transitioning microbolometerpixel membrane structure may be employed for individual detectorelements (e.g., of a focal plane array of an uncooled infrared detectoror any other type of radiation detector that includes a microbolometermembrane structure). In such an embodiment, the microbolometer pixelmembrane structure of each detector element may be configured to absorbradiation less efficiently as its temperature increases, thus reducingits ultimate maximum temperature. Such an optically transitioningmicrobolometer pixel membrane structure configuration may be implementedto enable a detector element to view a scene that includes undesiredradiation wavebands (e.g., such as direct sunlight), while at the sametime protecting the detector element from temporary or permanent damage.The microbolometer pixel membrane structure may be disposed to absorbradiation over a silicon ROIC wafer that has a metal reflector on theROIC surface to form a tuned cavity, with distance from the membrane tothe mirrored reflector and sheet resistance of the semi-transparentconductive material of the membrane being two factors affecting cavityabsorption efficiency.

Examples of optically transitioning materials that may be employed aspart of an optically transitioning microbolometer pixel membranestructure or other type of thermal detector element component include,but are not limited to, thermochromic or phase shifting/phasetransitioning materials (e.g., thermochromic or phase transitioningcompositions of germanium-antimony-tellurium (GST orGe_(X)Sb_(Y)Te_(Z)), vanadium oxide (VO, VO₂, V_(n)O_(2n-1) such asV₂O₃), tungsten-doped vanadium oxide (W:VO_(X) such as W:VO₂, andstoichiometric variants), niobium oxide, tantalum oxide, Ti₂O₃, Fe₃O₄,Mo₉O₂₆, etc.) that act to selectively transmit and/or absorb radiationdepending on temperature. As described elsewhere herein, composition ofsuch materials may be varied (e.g., by doping with other materials suchas tungsten, aluminum and/or manganese) to tailor the optical transition(e.g., phase transition) characteristics of the material as a functionof temperature.

In another exemplary embodiment, uncooled infrared detectors and focalplane arrays may be provided in which an optically transitioning filterelement may be suspended over a corresponding microbolometer pixelmembrane structure of an infrared detector element, i.e., a separatesuspended optically transitioning filter element may be provided thatcorresponds to each of multiple detector elements of a focal plane array(FPA) in a one-to-one relationship (i.e., a separate optical elementprovided for each respective detector element of the FPA such that eachoptical element is suspended over and above radiation detectioncircuitry of the respective detector element and is not suspended overand above the radiation detection circuitry of other adjacent detectorelements, and such that each optical element only filters or focusesradiation destined for the underlying radiation detection circuitry ofthe respective radiation detector element and not any other radiationdetector elements of the FPA). An optically transitioning filter elementmay be structurally attached by an electrically and/or thermallyinsulating interconnect to existing metal interconnects. In this way,the installation of the optically transitioning filter elementsubstantially does not impact the thermal mass or degrade the thermaltime constant of the microbolometer pixel structure and does not requireany additional device real estate area beyond the area originallyconsumed by the microbolometer pixel structure interconnects so as topreserve the original infrared absorbing fill factor of themicrobolometer pixel structure.

Pixel-level optically transitioning filter elements may be employed toachieve real time filtering of radiation in an active manner withoutadding significant optical crosstalk effects, without degradation (i.e.,increasing) of the detector thermal time constant, and/or in a mannerthat allows for thermal response tuning independent of the detectorpixel. Such active pixel-level filtering may be implemented by providinga multi-level structure that includes a thermally and electricallyisolated optically transitioning filter element that is suspended over amicrobolometer pixel membrane structure of a corresponding infrareddetector element, i.e., in a one-to-one relationship (i.e., one opticalelement provided for each underlying detector element). Such anembodiment may be advantageously implemented to provide opticallytransitioning pixel-level filter elements to enable one or more featuressuch as spectral infrared radiation detection and/or selective radiationimmunity.

Optically transitioning filter elements disclosed herein may becontrolled by the properties of a selected optically transitioningmaterial or materials, i.e., the optically transitioning materialtransmits radiation at temperatures below the transition temperature ofthe material while it reflects at one or more specific radiationwavelength bands at temperatures above the transition temperature of thematerial. Since the thermal response is a result of the thermal flux ofthe radiation and the thermal isolation of the filter element (e.g.,including optically transitioning filter or combination of opticallytransitioning and passive filters), it therefore can be controlled withthermal isolation structures/forms. In this regard, an opticallytransitioning filter element may be suspended by one or more low thermalconductivity or thermally insulating support interconnect/s and thermalisolation structures/forms in a manner such that the opticallytransitioning filter element substantially does not affect the thermaltime constant of the underlying microbolometer pixel structure.Furthermore, the thermal isolation structure/forms may be implemented toallow for the tuning of the thermal response of the opticallytransitioning filter element to the flux of the radiation in a mannerthat is independent of the response of the underlying microbolometerpixel structure to the radiation flux.

Examples of applications for a detector element that includes anoptically transitioning filter element include, but are not limited to,partially or fully shielding underlying radiation detector circuitry(e.g., infrared microbolometer pixel membrane, charge coupled device(CCD) detector pixel, millimeter wave detector pixel, CMOS detectorpixel, etc.) from direct solar radiation or other undesired radiationwavebands. Such exposure may occur, for example, when a detector deviceincluding the detector element is inadvertently pointed at the sun, andmay result in permanent damage to the detector element or formation of alatent image that may remain on the detector element for an extendedperiod of time. In another exemplary embodiment, one or more individualdetector elements may be provided with optically transitioning filterelements that partially or fully shield underlying radiation detectorcircuitry from different levels of radiation than other detectorelements of the same focal plane array. For example, a first opticallytransitioning filter element may be provided to shield underlyingradiation detector circuitry of a first detector element from a firstlevel of infrared energy (e.g., energy having less than or equal toabout 5 microns wavelength) while a second optically transitioningfilter element may be provided to shield underlying radiation detectorcircuitry of an adjacent second detector element from a second level ofinfrared energy (e.g., energy having less than or equal to about 7microns wavelength).

To prevent such damage, individual detector elements (e.g., of a focalplane array) may be provided in one exemplary embodiment with opticallytransitioning filter elements that are composed of an opticallytransitioning material that remains substantially transmissive todesired radiation wavebands (e.g., indirect reflected sunlight,artificial room lighting, etc.), but that becomes substantiallynon-transmissive or opaque to undesired radiation wavebands (e.g.,direct sunlight, electrical arc radiation, etc.). In such an embodiment,the optically transitioning filter may be configured in a manner that itsufficiently dissipates heat when exposed to desired radiation wavebandsat normal operating temperature such that the optically transitioningmaterial remains below its optical transition (e.g., phase transition)temperature and in a corresponding substantially transmissive state, butalso in a manner such that exposure to radiation of an undesiredbandwidth generates sufficient heat that cannot be dissipated rapidlyenough to prevent the temperature of the phase transitioning materialfrom increasing above its optical transition (e.g., phase transition)temperature and becoming at least partially non-transmissive or opaqueto the undesired radiation. In this way, the optically transitioningfilter acts to transmit desired radiation to the underlying detectorcircuitry, but automatically shields the underlying detector circuitryfrom the undesired radiation.

The disclosed optically transitioning filter elements may be optionallycombined in one exemplary embodiment with passive optical elementsdisclosed elsewhere herein (e.g., spectral filters, polarizing filters,microlens, etc.) to enable other features such as polarimetric infrareddetection. Specific examples of applications for which the disclosedoptically transitioning pixel-level filtering may be employed include,but are not limited to, providing solar immunity for infrared detectors,providing temperature dependent spectral radiation selectivity,scientific applications (e.g., study of astronomical light sources,study of interstellar manner, etc.).

In one exemplary embodiment, a multi-layer structure may be providedthat includes an optically transitioning filter element layer that isoptionally combined with other layers of optically transitioning,passive or combination passive/optically-transitioning optical elementsthat each filter different wavelengths. Whether provided alone or incombination with other suspended optical elements, a multilayeroptically transitioning structure (or other type of suspended opticalelement disclosed herein) may be provided that is thermally isolated andpositioned in close proximity (e.g., less than about 10 microns in oneembodiment, less than about 10 microns and greater than or equal toabout 1 micron in another embodiment, from about 1 micron to about 10microns in another embodiment, less than or equal to about 5 microns inanother embodiment, from about 1 micron to about 5 microns in anotherembodiment; about 2 microns in another embodiment, less than or equal toabout 2 microns in another embodiment; and from 1 micron to about 2microns in another embodiment) to an underlying microbolometer pixelstructure or other type of detector circuitry to reduce cross talkbetween adjacent pixels of a FPA.

In another exemplary embodiment, an optically transitioning filterelement may be monolithically fabricated with an underlying detectorpixel, or may be separately fabricated and then attached or otherwiseassembled to a detector element. However, in one exemplary embodiment,pixel-level optically transitioning filter elements may be“monolithically fabricated” in-situ, i.e., the layers or otherstructural elements of an optically transitioning filter element isfabricated in place over the microbolometer pixel membrane structure ofa corresponding infrared detector element, rather than being fabricatedseparately from other components of the infrared detector element andthen assembled to the infrared detector element.

In one exemplary embodiment, a monolithic optically transitioning filterelement may be fabricated using modern photolithographic techniques sothat precise registration and alignment of the optically transitioningfilter element over a bolometer pixel is achieved. In anotherembodiment, the suspended optically transitioning filter element may bedisposed in sufficiently close proximity (e.g., about 2 microns in oneembodiment, less than or equal to about 2 microns in another embodiment)to the underlying detector circuitry (e.g., microbolometer pixelstructure) to minimize or substantially eliminate cross talk betweenadjacent pixels of a FPA. However, a suspended optically transitioningfilter element may be disposed at any other greater or lesser distancerelative to a microbolometer pixel membrane structure, e.g., as may beappropriate to optical focusing requirements for a given application.

Among the infrared applications for which the disclosed radiationdetectors and focal plane arrays may be employed include, but are notlimited to, spectroscopic polarimetric infrared imaging applicationssuch as identification of man-made objects (e.g., vehicles, armor, etc.)in a cluttered background of natural objects such as trees, vegetationor concealment materials, identification of non-polarizing layers onpolarizing surfaces (e.g., ice on highways, ice on aircraft wings,etc.), scientific applications (e.g., study of astronomical lightsources, interstellar matter, etc.), etc.

In one respect, disclosed herein is a thermal detector element includingan optically transitioning thermal absorption structure, the thermaldetector element being configured to sense radiation falling incidentthereon by measuring at least one property of the thermal absorptionstructure that changes value with temperature, the thermal absorptionstructure being provided with one or more components that include atleast one optically transitioning material.

In another respect, disclosed herein is a focal plane array assembly,including a plurality of individual thermal detector elements arrangedas an array, at least a portion of the plurality of individual thermaldetector elements including an optically transitioning thermalabsorption structure and being configured to sense radiation fallingincident thereon by measuring at least one property of the thermalabsorption structure that changes value with temperature, the thermalabsorption structure being provided with one or more components thatinclude at least one optically transitioning material. A wafer-levelpackaged focal plane array assembly is also disclosed that includes: adevice wafer that includes the focal plane array assembly; and a lidwafer, the lid wafer being at least partially transmissive of theincident radiation and being assembled to the device wafer such that thelid wafer allows the incident radiation to reach the focal plane arrayassembly through the lid wafer.

In another respect, disclosed herein is a method of making a focal planearray assembly, including forming a plurality of individual thermaldetector elements arranged as an array, each of the plurality ofindividual detector elements including an optically transitioningthermal absorption structure and being configured to sense radiationfalling incident thereon by measuring at least one property of thethermal absorption structure that changes value with temperature, thethermal absorption structure being provided with one or more componentsthat include at least one optically transitioning material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a perspective view of a conventional prior artuncooled infrared detector element.

FIG. 2 illustrates a perspective view of an uncooled infrared detectoraccording to one exemplary embodiment of the disclosed apparatus andmethods.

FIG. 3 illustrates a perspective view of an uncooled infrared detectorelement according to one exemplary embodiment of the disclosed apparatusand methods.

FIG. 4 illustrates a top view of a subarray according to one exemplaryembodiment of the disclosed apparatus and methods.

FIG. 5 illustrates a top view of a subarray according to one exemplaryembodiment of the disclosed apparatus and methods.

FIG. 6 illustrates a top view of a focal plane array according to oneexemplary embodiment of the disclosed apparatus and methods.

FIG. 7 illustrates a top view of a subarray according to one exemplaryembodiment of the disclosed apparatus and methods.

FIG. 8 illustrates absorptance versus wavelength according to oneexemplary embodiment of the disclosed apparatus and methods.

FIG. 9 illustrates polarizer transmittance versus wavelength accordingto one exemplary embodiment of the disclosed apparatus and methods.

FIG. 10 illustrates absorptance versus wavelength according to oneexemplary embodiment of the disclosed apparatus and methods.

FIG. 11 illustrates polarizer transmittance versus wavelength accordingto one exemplary embodiment of the disclosed apparatus and methods.

FIG. 12 illustrates a perspective view of an uncooled infrared detectorelement according to one exemplary embodiment of the disclosed apparatusand methods.

FIG. 13 illustrates a top view of a subarray according to one exemplaryembodiment of the disclosed apparatus and methods.

FIG. 14 illustrates a top view of a focal plane array according to oneexemplary embodiment of the disclosed apparatus and methods.

FIG. 15 illustrates a top view of a subarray according to one exemplaryembodiment of the disclosed apparatus and methods.

FIG. 16 illustrates a side cross-sectional view of a material layersequence of an uncooled infrared detector element according to oneexemplary embodiment of the disclosed apparatus and methods.

FIG. 17 illustrates absorptance versus wavelength according to oneexemplary embodiment of the disclosed apparatus and methods.

FIG. 18A illustrates absorptance versus wavelength according to oneexemplary embodiment of the disclosed apparatus and methods.

FIG. 18B illustrates absorptance versus wavelength according to oneexemplary embodiment of the disclosed apparatus and methods.

FIG. 19 illustrates an exploded perspective view of an uncooled infrareddetector element according to one exemplary embodiment of the disclosedapparatus and methods.

FIG. 20 illustrates a perspective view of an uncooled infrared detectorelement according to one exemplary embodiment of the disclosed apparatusand methods.

FIG. 21 illustrates a cross sectional view of an uncooled infrareddetector element according to one exemplary embodiment of the disclosedapparatus and methods.

FIG. 22 illustrates a top view of a microbolometer pixel membranestructure according to one exemplary embodiment of the disclosedapparatus and methods.

FIG. 23 illustrates a cross sectional view of an uncooled infrareddetector element according to one exemplary embodiment of the disclosedapparatus and methods.

FIG. 24 illustrates a perspective view of an uncooled infrared detectorelement according to one exemplary embodiment of the disclosed apparatusand methods.

FIG. 25 illustrates a perspective view of an uncooled infrared detectorelement according to one exemplary embodiment of the disclosed apparatusand methods.

FIG. 26 illustrates a top view of a subarray according to one exemplaryembodiment of the disclosed apparatus and methods.

FIG. 27 illustrates a perspective view of a subarray according to oneexemplary embodiment of the disclosed apparatus and methods.

FIG. 28 illustrates a perspective view of a subarray according to oneexemplary embodiment of the disclosed apparatus and methods.

FIG. 29 illustrates a perspective view of an uncooled infrared detectorelement according to one embodiment of the disclosed apparatus andmethods.

FIG. 30 illustrates a side cross-sectional view of a material layersequence of an uncooled infrared detector element according to oneexemplary embodiment of the disclosed apparatus and methods.

FIG. 31 illustrates sheet resistance (Rsh) versus temperature forvarious doped compositions of vanadium oxide.

FIG. 32 illustrates temperature dependence of parallel Rsh infraredlight absorption efficiency for various doped compositions of vanadiumoxide and titanium-aluminum.

FIG. 33 illustrates a simplified side cross-sectional view of anuncooled infrared detector element according to one exemplary embodimentof the disclosed apparatus and methods.

FIG. 34 illustrates a simplified side cross-sectional view of anuncooled infrared detector element according to one exemplary embodimentof the disclosed apparatus and methods.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 2 is a diagrammatic perspective view of an uncooled IR detector 210which may be configured according to one exemplary embodiment to sensethermal energy and output electrical signals representative of atwo-dimensional image of that sensed thermal energy. In this embodiment,the IR detector 210 includes a focal plane array (FPA) 212 disposed on asubstrate 216. The substrate 216 includes read out integrated circuitry(ROIC) and FPA 212 may be uncooled. The ROIC may be configured tointegrate thermally induced electrical signals from the suspendedmicrobolometer detector pixel element of each IR detector element 300 inthe focal plane array 212. Each suspended microbolometer detector pixelelement may include a suspended monolithic filter as will be furtherdescribed herein. As further shown in FIG. 2, an optional thermalelement 217 (e.g., active heat sink) may be provided on the side of thesubstrate 216 opposite from the focal plane array 212, in order to serveas a form of controlled heat sink which may be used, for example, tomaintain the operating temperature of integrated circuit substrate 216within a temperature range which may be predefined. However, it will beunderstood that thermal element 217 does not have to be present, andthat no form of temperature stabilization is required.

In one exemplary embodiment, the focal plane array 212 may include aplurality of IR detector elements 300 that are arranged in atwo-dimensional array, with each detector element 300 corresponding to arespective pixel in each image detected by the IR detector 210. In oneexemplary configuration, focal plane array 212 may include 76,800 IRdetector elements 300, which are arranged in a 320 by 240 array,although a focal plane array may be of any other size depending asneeded or desired for a particular application, e.g., ranging in sizefrom greater than or equal to an 80×60 array up to megapixel-sizedarrays. For clarity, however, FIG. 2 diagrammatically depicts only about140 detector elements. It will be recognized that the total number ofdetector elements 300 in the focal plane array 212 may be larger orsmaller. FIG. 2 shows the detector elements 300 arranged in atwo-dimensional array. Examples of two-dimensional array configurationsinclude, but are not limited to, arrays having a shape that isrectangular, octagonal, hexagonal, circular, etc. It will be understoodthat detector elements may alternatively be arranged in aone-dimensional (e.g., straight or curved line of mono-pixels) array, ormay be provided at arbitrary locations that do not conform to a specificpattern.

In one embodiment, a focal plane array 212 may be formed and placed in asingle vacuum package to form a vacuum-packaged pixel array structurefor thermal imaging. In this embodiment, the IR detector elements 210are discrete devices detecting thermal energy in a specific portion of atarget (scene) area. For example, IR detector elements 210 may be formedon a device wafer and then sealingly assembled with a lid wafer that isat least partially transmissive of infrared radiation (i.e., having atleast some infrared radiation transmission characteristics) in thepresence of a vacuum to sealingly contain a vacuum therebetween,although non-vacuum packaged device wafer/lid wafer package combinationsare also possible, as are non-IR FPAs and transmissive lids therefor.

Examples of technology (e.g., including materials and/or configurationsfor detectors, focal plane arrays, microbolometer membranes, andpackaging thereof) with which the various disclosed embodimentsdisclosed herein may be implemented may be found in U.S. Pat. No.7,459,686; U.S. Pat. No. 6,777,681; U.S. Pat. No. 6,690,014; and U.S.patent application Ser. No. 12/799,627, entitled “PIXEL STRUCTURE FORMICROBOLOMETER DETECTOR” by Hanson which is concurrently filed on thesame day as the present application, each of the foregoing beingincorporated herein by reference in its entirety. Besides IR detectorelements 300, it will be understood that the configuration of focalplane array 212 may alternatively or additionally include a plurality ofIR detector elements 1200, 1900, 2400 and/or 2500 (which are describedfurther herein) that are arranged in a two-dimensional array in asimilar manner as IR detector elements 300.

FIG. 3 illustrates one exemplary embodiment of an uncooled infrareddetector element 300 that includes an optical element in the form of amonolithic polarizer filter 322 suspended at a distance of from about 1micron to about 3 microns over and above a microbolometer pixel membranestructure 312 by two optical element support interconnects 320 thatextend down and are substantially aligned to and supported by the top ofthermally and electrically conductive interconnects 308 (e.g., titanium,tungsten, titanium-tungsten (TiW), aluminum, or any other suitablythermally and electrically conductive material) of microbolometer pixelstructure 312). In one exemplary embodiment, conductive interconnects308 may be configured in hollow tubular form (e.g., cylindrical, square,conical, etc.), with sidewalls of about 1750 angstroms thick, althoughsolid interconnects or interconnects having greater or lesser wallthicknesses may be alternately employed. Conductive interconnects 308are in turn coupled to readout integrated circuitry of the supportingsemiconductor substrate 216 by electrically conductive input pads 314which may be made of aluminum or other suitably electrically conductivematerial. Conductive interconnects 308 are configured in one embodimentto supply electrical connection between microbolometer pixel membranestructure 312 and circuitry of substrate 216.

As shown in FIG. 3, microbolometer pixel membrane structure 312 includesa thin absorber layer (e.g., from about 1000 Angstroms to about 4000Angstroms thick) of thermally absorbing resistive membrane material(e.g., such as amorphous silicon (a-Si), amorphous silicon geranium(a-SiGe), Vanadium Oxide (VOx), etc.) together with an infraredabsorbing thin metal thermally-electrically active layer (e.g., titanium(Ti), titanium aluminum (TiAl), titanium nitride (TiN), nickel, etc.),and supporting material (e.g., silicon nitride, silicon oxide, siliconoxynitride, etc.). Further detail on one exemplary embodiment ofcomponents of microbolometer pixel membrane structure 312 may be foundin FIGS. 19-22 herein. In one exemplary embodiment, the microbolometerpixel membrane structure 312 may be suspended about 2 microns abovesilicon semiconductor substrate 216 by relatively long thermal isolationlegs 316 that are electrically connected to the ROIC of thesemiconductor substrate 216 by the metal interconnects 308 viaconductive input pads 314, although a microbolometer pixel membranestructure 312 may be suspended greater than or less than about 2 micronsabove a substrate (e.g., such as above silicon semiconductor substrate216 or above a reflector layer disposed on a silicon semiconductorsubstrate) in other embodiments. Conductive interconnects 308 areelectrically coupled to pass current across the thermally-electricallyactive layer of microbolometer pixel membrane structure 312 to allowROIC of the semiconductor substrate 216 to measure changing electricalproperties of the thermally-electrically active layer as varying levelsof infrared radiation is absorbed by microbolometer pixel membranestructure 312.

A reflector 318 (e.g., reflective metal such as aluminum, etc. or othersuitably reflective non-metal material) for reflecting IR radiation maybe optionally disposed as shown on the surface of the supportingsubstrate 216 and ROIC to form a resonant cavity structure to enhanceinfrared absorptance in the membrane of the suspended microbolometerpixel structure 312. In one exemplary embodiment, pixel geometry may besquare-shaped and the pixel area may range from about 12 um×12 um toabout 100 um×100 um, although pixel shapes other than square and/orpixel sizes smaller than 12 um×12 um or greater than 100 um×100 um maybe employed in other embodiments. As shown, microbolometer pixelmembrane structures 312 has been implemented in a diffractive resonantcavity (DRC) configuration with material being removed from the membranematerial of suspended microbolometer pixel membrane structure 312 in asquare or rectangular grid pattern to form openings 310 defined byconductive element portions 306 that are oriented both parallel andorthogonal to the path of current flow between the electrical contacts302 and 304. Openings 310 may be dimensioned to be small compared to theradiation wavelength of interest. It will be understood that in otherembodiments, openings may be defined in a microbolometer pixel membranestructure using pattern shapes other than square or rectangular.

Examples of IR detectors and focal plane array technology with which thedisclosed monolithic optical elements may be implemented in a suspendedconfiguration relative to a microbolometer membrane structure may befound in U.S. Pat. No. 7,459,686; U.S. Pat. No. 6,777,681; and U.S. Pat.No. 6,690,014, and U.S. patent application Ser. No. 12/799,627, entitled“PIXEL STRUCTURE FOR MICROBOLOMETER DETECTOR” by Hanson which isconcurrently filed on the same day as the present application, each ofthe foregoing being incorporated herein by reference in its entirety.

Still referring to FIG. 3, monolithic polarizer filter 322 may be of anymaterial of a rigidity and thickness suitable for patterning. In oneexemplary embodiment, polarizer filter 322 may be about 2500 Angstromsthick aluminum metal. Other materials that may be employed to formpolarizer filter 322 include, but are not limited to, silicon nitride,silicon dioxide, etc. As shown monolithic polarizer filter may bepatterned into a linear diffraction grating that includes elongatedgrating elements (wires) 324 that are separated by elongated spaces 326.It will be understood that dimensions of such a diffraction grating orother polarizer filter design may vary according to the desiredfiltering characteristics to be imparted by polarizer filter 322. Forexample, in one exemplary embodiment grating elements 324 may be 0.5micron wide wires that are spaced apart by a 1 micron center to centerperiod. In another exemplary embodiment, grating elements 324 may be 1micron wide wires that are spaced apart by a 2.5 micron center to centerperiod. Other grating element widths and/or period spacings arepossible. In this regard, grating element width and period may beselected based on factors such as spectrum or spectra to be filtered,required polarizer efficiency, etc.

Monolithic polarizer filters 322 may be formed using any techniquesuitable for monolithically fabricating polarizing filters 322 alignedin place above corresponding microbolometer pixel membrane structures312. In this regard, an array of microbolometer pixel membranestructures 312 may be coated with a second layer of polyimide (e.g.,about 1 to about 3 microns or any other suitable thickness) or othersuitable second sacrificial layer after the layers of membranestructures 312 are formed. The layers for the corresponding monolithicpolarizer filters 322 may then be formed above the second sacrificiallayer, and interconnects 320 may be made through vias in the secondsacrificial layer to support the monolithic polarizer filters 322.Thereafter, the second sacrificial layer may be removed from beneath thepolarizer filters 322 by undercutting, e.g., by oxygen plasma isotropicetch. An oxygen plasma isotropic etch may be employed, for example, toetch beneath (undercut) the suspended portions of the monolithicpolarizer filters 322. Since electrically and/or thermally insulatinginterconnects 320 connect directly to the conductive interconnects 308that support the first level microbolometer pixel membrane structure312, the polarizer filter structure 322 does not add thermal mass to themicrobolometer pixel membrane structure 312. It will be understood thateither or both of interconnects 320 and/or monolithic polarizer filters322 may be electrically insulating such that a current path is notformed that bypasses the microbolometer pixel membrane structure 312.

Optical element support interconnects 320 for monolithic polarizerfilter 322 may be of any material and thickness suitable for supportingpolarizer 322 and meeting thermal and electrical property requirements.For example, in one exemplary embodiment an electrically conductive(e.g., aluminum) polarizer filter 322 may be supported by electricallyinsulating (e.g., silicon nitride, silicon oxide, silicon oxynitride, orother suitable electrically insulating material) optical element supportinterconnects 320, although it will be understood that electricallyconductive optical element support interconnects 320 may be employedwhere polarizer filter 322 is non-electrically conductive or where aseparate electrically insulating material is present to preventoccurrence of an electrical short between electrically conductiveinterconnects 308 through polarizer filter structure 322. Opticalelement support interconnects 320 may be selectably fabricated to aheight that matches the second sacrificial layer (e.g., polyimide layer)to achieve desired spacing (e.g., about 2 microns), and may be providedas plug interconnects, etc. In one exemplary embodiment, polarizerfilter 322 and optical element support interconnects 320 may befabricated so that insulating material (e.g., silicon nitride) used toform the insulating interconnects optionally remains over (i.e.,overlays) a portion or substantially all of the entire upper surface ofthe monolithic polarizer filter 322 (e.g., 2500 Angstroms thick aluminummetal grating). However, it is not necessary that such an insulatingmaterial overlay monolithic polarizer filter 322. In another embodiment,each of optical element support interconnects 320 may be formed from adownwardly-protruding extension of the filter material. In one exemplaryembodiment, each of optical element support interconnects 320 may beconfigured in hollow tubular form (e.g., cylindrical, square, etc.),having a wall thickness of about 1750 angstroms, although solidinterconnects or interconnects having greater or lesser wall thicknessesmay be alternately employed.

In one exemplary embodiment, adjacent uncooled infrared detectorelements 300 of a focal plane array 212 may be provided withdifferently-configured monolithic polarization filters 322, e.g., sothat adjacent infrared detector element pixels 300 absorb differentpolarization components of the scene infrared radiation. For example,FIG. 4 illustrates a 2×2 subarray 400 of four adjacent microbolometerpixel membrane structures 312 a-312 d, shown without overlying polarizerfilter structures 322. Subarray 400 may form a part of a larger FPA 212,e.g., such as a 30 micron pixel 320×240 amorphous silicon microbolometerFPA 212. In this embodiment, no openings 310 are defined in pixelmembrane structures 312 a-312 d, although such openings may be presentin other embodiments.

FIG. 5 illustrates a 2×2 subarray 500 that includes polarizer filterstructures 322 a-322 d in place overlying the correspondingmicrobolometer pixel membrane structures 312 a-312 d of 2×2 subarray400. As shown in FIG. 5, the polarizer filter structures of FIG. 5include horizontal monolithic grating structure 322 a, +45° monolithicgrating structure 322 b, −45° monolithic grating structure 322 c, andvertical monolithic grating structure 322 d. In the illustratedembodiment of FIG. 5, four polarizations (corresponding to four Stokesvectors) are included in 2×2 subarray 500. However, it will beunderstood that the embodiment of FIG. 5 is exemplary only, and that anyother combination of two or more differently-configured monolithicpolarization filters 322 may be employed in the same FPA 212, in aregular or irregular pattern, or in a random arrangement. FIG. 6illustrates the pattern of 2×2 subarray 500 of FIG. 5 repeated to form alarger polarized imaging focal plane array 550.

In an alternative embodiment, one or more microbolometer pixel membranestructures 312 of a focal plane array 212 may be left uncovered with nooverlying polarizer filter structure 322. For example, FIG. 7illustrates a 2×2 subarray 600 similar to subarray 500 of FIG. 5, withthe exception that the +45° polarizer filter structure 322 b has beenomitted so that microbolometer pixel membrane structure 312 b absorbsunfiltered and randomly polarized scene radiation. It will be understoodthat FIG. 7 is exemplary only, and that any one or more polarizer filterstructures 312 may be omitted, and/or any one or more microbolometerpixel membrane structures 312 may be left uncovered, in a regular orirregular pattern, or in a random arrangement.

FIGS. 8 and 9 illustrate polarization sensitivity for an uncooledinfrared detector element 300 of one exemplary embodiment having apolarizer filter 322 in the form of a monolithic grating structurefabricated about 2 microns above a microbolometer pixel membranestructure 312. In this embodiment, the grating structure 322 has gratingelements 324 that are 0.5 micron wide wires spaced apart by a 1.25micron center to center period. In particular, FIG. 8 illustratesabsorptance plotted versus wavelength, and shows absorptance of about 2%in parallel mode that may be mostly attributed to the polarizer filter322. FIG. 9 illustrates polarizer transmittance versus wavelength, andshows perpendicular transmittance that is greater than 95% and paralleltransmittance of less than 0.4%. This yields a discrimination ratio ofgreater than about 237 (>95/0.4). Absorptance in grating is about 1.5%in parallel mode.

FIGS. 10 and 11 illustrate polarization sensitivity for an uncooledinfrared detector element 300 of one exemplary embodiment having apolarizer filter 322 in the form of a monolithic grating structurefabricated 2 microns above a microbolometer pixel membrane structure312. In this embodiment, the grating structure 322 has grating elements324 that are 1 micron wide wires spaced apart by a 2.5 micron center tocenter period. In particular, FIG. 10 illustrates absorptance plottedversus wavelength, and shows absorptance of about 2% in parallel modethat may be mostly attributed to the polarizer filter 322. FIG. 11illustrates polarizer transmittance versus wavelength, and showsperpendicular transmittance that is about 95% and parallel transmittanceof about 2.5%. This yields a discrimination ratio of about 38 (>95/2.4).Absorptance in grating is about 1.5% in parallel mode.

FIG. 12 illustrates one exemplary embodiment of an infrared detectorelement 1200 that includes an optical element in the form of amonolithic spectral filter 1222 suspended by two optical element supportinterconnects 320 (e.g., at a distance of about 1 to 3 microns) over andabove a microbolometer pixel membrane structure 312 and associatedsilicon semiconductor substrate 216 and optional reflector 318 of thesame type described in relation to FIG. 3. In this regard,microbolometer pixel membrane structure 312 is itself suspended abovesilicon semiconductor substrate 216 by relatively long thermal isolationlegs 316 that are electrically connected to the ROIC of thesemiconductor substrate 216 by the metal interconnects 308 viaconductive input pads 314. As with the embodiment of FIG. 3, amicrobolometer pixel membrane structure 312 may be suspended greaterthan or less than about 2 microns above silicon semiconductor substrate216 in other embodiments. It will be understood that a plurality ofinfrared detector elements 1200 may be employed in a manner similar toinfrared detector elements 300 to form a focal plane array focal planearray 212 such as illustrated and described in relation to FIG. 2.

In one embodiment of FIG. 12, monolithic spectral filter 1222 may beconfigured as a stack of optical thin film layers characterized by athickness and refractive index that serves a spectral filteringfunction, e.g., such as forming a spectral band pass which confines theradiation absorbed by microbolometer pixel membrane structure 312 to bewithin a specific spectral band pass. Other examples of spectral filtertypes include, but are not limited to, high pass filter, low passfilter, notch filter (e.g., to filter out laser radiation), etc.

For example, spectral filter 1222 may be configured in one exemplaryembodiment for passing LWIR radiation using the film layers 1620, 1622and 1624 and corresponding thicknesses of Table 1 below, and in anotherexemplary embodiment for passing MWIR radiation using the film layers1620, 1622 and 1624 and corresponding thicknesses of Table 2 below. Inthe first of such embodiments, overall thickness of filter 1222 may beabout 0.17 microns and may limit radiation passed for absorption bymicrobolometer pixel membrane structure 312 to those absorbed long tovery long wave wavelengths shown in FIG. 17. In the second of suchembodiments, overall thickness of filter 1222 may be about 0.13 micronsand may limit radiation passed for absorption by microbolometer pixelmembrane structure 312 to those absorbed midwave wavelengths shown inFIG. 18. It will be understood that in other embodiments, spectralfilter 1222 may be configured with other thickness and refractive indexvalues to limit absorption of other wavelengths. Alternatively, spectralfilter 1222 may be configured to serve a band stop function in which theradiation in the spectral band stop is substantially prevented frombeing absorbed by the underlying membrane structure 312. In any case,optical element support interconnects 320 may be configured from similarmaterials (e.g., electrically insulating SiN) and with similardimensions as previously described in relation to FIG. 3, and may in oneembodiment be formed from downwardly protruding extensions of the filterstack layers. Optical element support interconnects 320 may be disposedon electrically conductive interconnects 308 (e.g., TiW) as shown tominimize any addition to the pixel thermal mass or change to the pixeltime constant.

In yet another embodiment, monolithic spectral filter 1222 may bereplaced with an overlying optical block structure that shields theunderlying microbolometer pixel membrane structure 312 substantiallycompletely from incoming radiation. This may be done to configure one ormore uncooled infrared detector elements 300 as “blind pixels” that mayserve as reference pixels when combined with non-blind infrared detectorelements 300 that act as active imaging pixels. In this regard, such“blind” reference pixels have a dark signal response that may be used asa baseline for measuring the response of the active imaging pixelsexposed to incoming radiation. Such an overlying optical blockembodiment structure may be of any material and/or configurationsuitable for substantially blocking all incoming radiation in thespectral band to which the underlying microbolometer pixel membranestructure 312 is designed to respond, e.g., a thin metal reflectivelayer, a reflecting dielectric layer stack, and absorbing metal, anabsorbing layer stack, etc. In such an embodiment, uncooled infrareddetector elements 300 with optical block structures may be randomly orsystematically distributed over microbolometer pixel membrane structures312 of FPA 212.

In an alternative embodiment, one or more microbolometer pixel membranestructures 312 of a focal plane array 212 may be covered with a spectralfilter structure 1222, while one or more other microbolometer pixelmembrane structures 312 of the same focal plane array 212 may be leftuncovered with no overlying spectral filter structure 1222. For example,FIG. 13 illustrates a 2×2 subarray 1300 in which only alternatingmicrobolometer pixel membrane structures 312 are provided withmonolithically integrated spectral filters 1222. In FIG. 13, a 2×2subarray 1300 of four adjacent microbolometer pixel membrane structures312 a-312 d as shown in FIG. 4 is overlain by alternating monolithicallyintegrated spectral filter structures 1222 a and 1222 d, leavingmicrobolometer pixel membrane structures 312 b and 312 c uncovered toreceive unfiltered scene radiation. It will be understood that FIG. 13is exemplary only, and that any one or more spectral filter structures1222 may be provided, and/or any one or more microbolometer pixelmembrane structures 312 may be left uncovered, in a regular or irregularpattern, or in a random arrangement. FIG. 14 illustrates the pattern of2×2 subarray 1300 of FIG. 13 repeated to form a larger pixel-levelspectrally filtered imaging focal plane array 1400 in which every othermicrobolometer pixel membrane structure 312 is provided with amonolithically integrated spectral filter structure 1222.

FIG. 15 illustrates another embodiment in which a 2×2 subarray 1500includes four adjacent microbolometer pixel membrane structures 312a-312 d of FIG. 4 that are overlain by respective alternating differenttypes of monolithically integrated spectral filter structures 1222. Inthis regard, a first type of spectral filter structures (1222 a and 1222d) are positioned adjacent a second type of spectral filter structures(1222 b and 1222 c). For example, in one exemplary embodiment, the firsttype of spectral filter structures may be mid-wave IR, and the secondtype of spectral filter structures may be long-wave IR, or sub-bands ofsuch. However, any other two or more different types of filterstructures may be so positioned over microbolometer pixel membranestructures 1222 of a focal plane array 212, in a regular or irregularpattern, or in a random arrangement.

FIG. 16 illustrates a side cross-sectional view of a material layersequence 1600 for one exemplary embodiment of an uncooled infrareddetector element 1200 that monolithically incorporates a spectral filter1222 such as described and illustrated in relation to FIG. 12. In oneembodiment, the layers of FIG. 16 may be formed by deposition on top ofa substrate 216 and ROIC 1602. As shown in FIG. 16, layer sequence 1600includes ROIC 1602 of substrate 216 with optional reflective aluminumlayer 318 disposed thereon. A first cavity 1606 separates substrate 216and reflective layer 318 from microbolometer pixel membrane structure312, which itself includes silicon nitride layers 1608 and 1612 with athermally-electrically active layer 1610 of amorphous silicontherebetween. Titanium absorber layer 1614 overlays silicon nitridelayer 1612, and is overlain itself by silicon nitride layer 1616 tocomplete microbolometer pixel membrane structure 312. The particularlayers illustrated in FIG. 16 for pixel membrane structure 312 areexemplary only, and may be varied in number, type and/or thicknesses toobtain the desired infrared detection characteristics. A second cavity1618 separates microbolometer pixel membrane structure 312 from spectralfilter 1222, which itself includes silicon nitride layers 1620 and 1624with amorphous germanium layer 1622 therebetween. Layers of spectralfilter 1222 define the spectral response of the composite spectralfilter structure and may be varied in number, type and/or thicknesses toobtain a desired spectral response.

Any technique suitable for monolithically fabricating spectral filters1222 in place above corresponding microbolometer pixel membranestructures 312 may be employed. For example, as previously described forthe polarizer filter embodiments, an array of microbolometer pixelmembrane structures 312 may be coated with a second layer of polyimideor other suitable second sacrificial layer (e.g., about 1 to 3 micronsor any other suitable thickness) after the layers of membrane structures312 are formed. The layers for the corresponding monolithic spectralfilters 1222 may then be formed above the second sacrificial layer, andinterconnects 320 may be made through vias in the second sacrificiallayer to support the spectral filter structures 1222. Thereafter, thelayers of the spectral filters 1222 may be etched down through thesecond sacrificial layer to the underlying membrane structures 312. Anoxygen plasma isotropic etch may be employed, for example, to etchbeneath (undercut) the suspended portions of the monolithic spectralfilters 1222. Since interconnects 320 connect directly to the conductiveinterconnects 308 that support the first level microbolometer pixelmembrane structure 312, the spectral filter structure 1222 does not addthermal mass to the microbolometer pixel membrane structure 312.

It will be understood that a similar material layer sequence as shown inFIG. 16 may be employed for microbolometer pixel membrane structures ofother optical element embodiments disclosed herein if desired, such asfor the uncooled infrared detector element 300 of FIG. 3 that includes amonolithic polarizer filter 322 suspended over and above themicrobolometer pixel membrane structure 312. In such cases, layers 1620,1622 and 1624 of spectral filter 1222 may be replaced withmaterials/layers appropriate to the type of optical element beingimplemented, e.g., replaced with an aluminum metal or other suitablepolarizer structure layer of monolithic polarizer filter 322 asdescribed and illustrated in relation to FIG. 3.

Table 1 illustrates exemplary dimensions for one possible embodiment ofa long wave infrared (LWIR) detector embodiment using the material layersequence 1600 of FIG. 16.

TABLE 1 LWIR Layer Structure Layer Thickness Layer Material(micrometers) 1318 Aluminum 0.25 1606 Vacuum Cavity 2.15 1608 SiliconNitride 0.03 1610 Silicon 0.065 1612 Silicon Nitride 0.04 1614 Titanium0.025 1616 Silicon Nitride 0.05 1618 Vacuum Cavity 0.8 1620 SiliconNitride 0.03 1622 Germanium 0.11 1624 Silicon Nitride 0.03

FIG. 17 illustrates broadband (long wave infrared LWIR to very long waveinfrared VLWIR) spectral response in the 8 to 12 micron long waveinfrared spectral band in terms of detector absorptance versuswavelength for the amorphous-silicon two-level detector embodimentaccording to the LWIR layer structure of Table 1.

Table 2 illustrates exemplary dimensions for one possible embodiment ofa mid wave infrared (MWIR) detector embodiment using the material layersequence 1600 of FIG. 16.

TABLE 2 MWIR Layer Structure Layer Thickness Layer Material(micrometers) 1318 Aluminum 0.25 1606 Vacuum Cavity 0.25 1608 SiliconNitride 0.03 1610 Silicon 0.06 1612 Silicon Nitride 0.04 1614 Titanium0.01 1616 Silicon Nitride 0.05 1618 Vacuum Cavity 1 1620 Silicon Nitride0.03 1622 Silicon 0.06 1624 Silicon Nitride T 0.04

FIG. 18A illustrates MWIR spectral response in the 3 to 5 micron midwave infrared spectral band in terms of detector absorptance versuswavelength for the amorphous-silicon two-level detector embodimentaccording to the MWIR layer structure of Table 2. It will be understoodthat the layer structure embodiments of Tables 1 and 2 are exemplaryonly, and that other materials, layer configurations and layerthicknesses may be employed as suitable or desired for the givenapplication.

FIGS. 19-22 illustrate an exemplary embodiment of a dual band detectorelement 1900 in which an optical element in the form of a microlensstructure 1902 is supported and suspended at a distance of about 3microns or other suitable distance over and above a microbolometer pixelmembrane structure 312 by two insulating optical element supportinterconnects 320 (e.g., SiN, etc.) that extend down and aresubstantially aligned to and anchored to the top of electricallyconductive interconnects 308 (e.g., TiW, aluminum, etc.) of a DRCmicrobolometer pixel structure 312 in a manner similar to thatpreviously described for certain other embodiments. As with thepreviously-described embodiments, conductive interconnects 308 are inturn coupled to readout integrated circuitry of the supportingsemiconductor substrate 216 by electrically conductive input pad 314which may be made of aluminum or other suitably electrically conductivematerial. In this exemplary embodiment, microbolometer pixel membranestructure 312 is provided with four openings 310 defined therein asshown. It will also be understood that a plurality of infrared detectorelements 1900 may be employed in a manner similar to infrared detectorelements 300 to form a focal plane array focal plane array 212 such asillustrated and described in relation to FIG. 2.

In the embodiments of FIGS. 19-22, openings 1960 are defined as shown inoptional metal reflector 318 (e.g., aluminum, etc.). Openings 1960 ofmetal reflector 318 are vertically aligned with openings 310 ofmicrobolometer pixel membrane structure 312, which are in turnvertically aligned with indium-gallium-arsenic (InGaAs) diode detectors1980 as shown. Other exemplary types of detectors 1980 include, but arenot limited to, silicon (Si) diode, germanium (Ge) diode, andsilicon-germanium (SiGe) diode detectors. In this embodiment, microlensstructure 1902 includes an array of four microlenses 1904 that eachfocus a combination of visible, near IR, and short IR spectrum (VNS)radiation (i.e., substantially all radiation from visible to short IR)through a corresponding opening 310 in reflector metal 318 and othercomponents of the DRC pixel structure onto a corresponding InGaAs, Si,Ge or SiGe detector 1980 imbedded in SiO₂ substrate 216 below embeddedmetal layers 1970 of ROIC 1602. Microlenses 1904 may be diffractive,refractive or a combination of refractive and diffractive by design. Itwill be understood that detectors configured for detecting other typesof radiation besides VNS radiation may be alternately employed in otherembodiments (e.g., short wave infrared or mid wave infrared radiationwhere microbolometer pixel membrane structure 312 is configured todetect LWIR radiation). As shown, each microlens structure 1902 isconfigured in this exemplary embodiment as concentrator lens having alarger optical area than the underlying optical areas of correspondingopening 310 in reflector metal 318 and detectors 1980.

Still referring to FIGS. 19-22, the pixel-level array of microlens 1904may be monolithically fabricated over and aligned to the DRCmicrobolometer pixel membrane structure 312 using any suitablemethodology. For example, in a manner similar to that previouslydescribed for polarizing and spectral filter embodiments, a secondpolyimide sacrificial layer (e.g., about 3 microns or any other suitablethickness) may be applied over the DRC pixel array. In this embodiment,interconnects 320 may be made through vias in the second polyimide layerto support the microlens array bridge structure 1902. Sinceinterconnects 320 connect directly to the conductive interconnects 308that support the first level microbolometer pixel membrane structure312, the microlens array bridge structure 1902 does not add thermal massto the microbolometer pixel membrane structure 312.

In one exemplary embodiment, the configuration of FIGS. 19-22 may beimplemented to allow simultaneous maximization of both LWIRmicrobolometer sensitivity and VNS InGaAs-diode sensitivity, forexample, in a multispectral pixel design incorporating a LWIR+MWIRamorphous silicon (a-Si) DRC pixel fabricated over a diode imager-basedVNS sensing pixel. Such a configuration may be used to simultaneouslymaximizing sensitivity in both spectral bands by transmitting VNSradiation through the LWIR DRC pixel structure with minimal loss andwithout corrupting the LWIR resonant cavity absorptance characteristicsof the DRC pixel. In one embodiment, ROIC 1602 may employ separatein-pixel integrating amplifiers to integrate the signal from two (e.g.,infrared and VNS) spectral bands.

In one exemplary implementation of the embodiment of FIGS. 19-22, a lowthermal mass a-Si LWIR DRC pixel design may be implemented that employsa microbolometer pixel membrane structure 312 that includes an infraredabsorbing grating structure 1998 (e.g., TiAl structure of about 200Angstroms or other suitable thickness) that is embedded in aSiN/a-Si/SiN membrane bridge structure 1996 suspended (e.g., at adistance of about 2 microns or other suitable distance) above metalcavity reflector layer 318. In one exemplary embodiment, the membrane1996 of microbolometer pixel membrane structure 312 may be about 1100Angstroms thick SiN/a-Si/SiN that is substantially VNS transparent evenwithout openings 310. However, transmittance may be further improved byproviding openings 310 to the membrane as well. For example, membraneopenings 310 of up to about 2.5 microns×about 2.5 microns may beprovided in one exemplary embodiment. However, membrane openings 310 ofgreater size are also possible, e.g., greater than about 3 microns×about3 microns, etc.

In one exemplary embodiment, the absorber structure of microbolometerpixel membrane structure 312 may include grating elements of about 1micron or other suitable width that are spaced on a period of about 5micron period and exhibit LWIR absorptance (e.g., of greater than about90%). In such an exemplary configuration, VNS transparent regions 1992of about 4 microns×4 microns that are free of metal grating elements1998 permit VNS radiation to efficiently transmit through microbolometerpixel membrane structure 312 in combination with openings 310 of about2.5 microns by about 2.5 microns. Use of an array of microlens 1904 tofocus VNS radiation through VNS transparent regions 1992 may be employedto advantageously maximize transmittance through the DRC microbolometerpixel membrane structure 312.

In one embodiment, cavity reflector metal 318 may be a highreflectivity, low loss aluminum metal layer that exhibits LWIRreflectivity of about 99.9% to ensure high absorptance in the DRC pixeldesign. In such an embodiment, high reflectivity may be substantiallymaintained for the cavity reflector 318 by minimizing the size ofopenings 1960 required to transmit the VNS radiation. This may beadvantageously accomplished by providing relatively small openings 1960(e.g., about 2 microns by about 2 microns) in cavity reflector metallayer 318 to transmit VNS radiation to embedded VNS detector diodes1980, in combination with leveraging the array of pixel-level microlens1904 to focus the VNS radiation into each of the relatively smallopenings 1960.

Referring to FIG. 21 in more detail, an array of ZnSe plano-convexmicrolenses 1904 may be provided that are configured to focus VNSradiation into individual openings 1960, through which the focusedradiation is transmitted through substrate 216 (e.g., SiO₂ substratehaving a refractive index n of about 1.5) to the embedded VNS detectordiodes 1980 as shown.

Table 3 lists one exemplary embodiment of modeled opticalcharacteristics that may be employed for components of detector element1900 of FIGS. 19-22 when configured in exemplary 17 micron square and 20micron square bolometer pixel sizes. It will be understood that Table 3includes modeled and exemplary data only, and that the characteristicsof a detector element 1900 may be varied from the data listed.

TABLE 3 Bolometer pixel pitch size 17 20 (microns) Field of view; FOV 2020 Plano-convex optic 8.5 10 diameter D (microns) Distance from optic to5 5 aperture 1₁ (microns) Effective f-number (f/#) 0.59 0.50 Refractiveindex for ZnSe; 2.7 2.7 n_(ZnSe) Effective Focal length 5 5 (EFL)(microns) = 1₁ Approximate plano-convex 8.5 8.5 lens radius of curvatureR = EFL * (n_(ZnSe) − 1) Approximate minimum lens 1.14 1.63 thickness(microns) = R − SQRT(R² − (D/2)²) ZnSe thickness (microns) 1.85 1.85 foranti reflection condition at 10 microns: λ/2n_(ZnSe) Reflector MetalAperture 1.74 1.74 size A (microns) = 2 * 1₁ * sin(FOV/2) Cavity spacingbetween 2 2 DRC membrane and reflector metal (microns) Opening size inDRC 3.4 4.0 membrane (microns) Distance from aperture to 5 5 Ge diode 1₂(microns) Oxide refractive index n_(oxide) 1.5 1.5 Size of Ge diode =4.34 4.34 2 * (1₂ * n + 1₁) * sin(FOV/2)

FIG. 23 illustrates an alternative exemplary embodiment of a dual banddetector element 1900 configured similar to the embodiment of FIG. 21,except that VNS diode (e.g., InGaAs) detectors 1980 are provided (e.g.,bonded or otherwise attached) at the top of substrate 216 immediatelybelow openings 1960 in reflector metal 318. In this exemplaryembodiment, an array of ZnSe plano-convex microlenses 1904 may beprovided that are configured to focus VNS radiation onto VNS detectordiodes 1980 positioned at individual openings 1960 as shown.

The integrated LWIR/VNS pixel design of the embodiments of FIGS. 19-23may be advantageously optimized in one embodiment to achieve VNSradiation transmission through the LWIR bolometer of greater than about80% while maintaining high LWIR infrared absorptance. Inclusion of thepixel-level microlens 1904 may be employed to provide an additionalmechanism to focus the VNS radiation through the DRC pixel onto the VNSradiation detectors 1980. Although the configuration of microlens 1904is depicted as a refractive optical element, any other suitable focusingoptical design configuration (e.g., broadband diffractive ordiffractive/refractive optic design fabricated by gray scalelithography) may be employed.

Focal plane arrays may be manufactured using any suitable or desirednumber of detector elements, and/or sizes of detector elements. Examplefocal plane array configurations include, but are not limited to, 17micron square detector element pixels configured in pixel array sizes of320×240, 640×480, 1024×768, etc. In one exemplary embodiment, 17 micronsquare detectors element pixels may configured in a 640×480 pixel arrayhaving f/l noise equivalent temperature difference (NETD)_(8-12um) ofabout 40 mK and a thermal time constant of about 10 milliseconds usinglow noise a-Si material with thermal coefficient of resistance (TCR) ofabout 3.1%/K. In one exemplary embodiment, microlenses 1904 employ a VNSantireflection coating.

It will be understood that the dual band detector element configurationsof FIGS. 19-23 are exemplary only. In this regard, a microlens ormicrolens array optical element may be suspended over and above themembrane material of a microbolometer pixel membrane structure of asingle band uncooled infrared detector element or a multi-band detectorcapable of detecting radiation in three or more different radiationbands (e.g., a microlens or microlens array optical element may besuspended over and above the membrane material of an uppermicrobolometer pixel membrane structure of a multi-band detector havingtwo or more stacked microbolometer pixel membrane structures havingaligned openings therein for transmitting radiation from the uppermostmicrobolometer pixel membrane structure to a substrate underlying thelowermost microbolometer pixel membrane structure).

FIG. 24 illustrates one exemplary embodiment of an uncooled infrareddetector element 2400 that includes an optically transitioning (e.g.,thermochromic or phase transitioning) filter element 2410 suspended bytwo optically transitioning filter element support interconnects 2402(e.g., at a distance of about 1 to 2 microns or other suitable distance)over and above a microbolometer pixel membrane structure 312 andassociated silicon semiconductor substrate 216 and optional reflector318 of the same type described in relation to FIG. 3. In this regard,microbolometer pixel membrane structure 312 may be itself suspended(e.g., at a distance of about 2 microns) above silicon semiconductorsubstrate 216 by relatively long thermal isolation legs 316 that areelectrically connected to the ROIC of the semiconductor substrate 216 bythe metal interconnects 308 via conductive input pads 314. As with theembodiment of FIG. 3, a microbolometer pixel membrane structure 312 maybe suspended greater than or less than about 2 microns above siliconsemiconductor substrate 216 in other embodiments. It will be understoodthat the disclosed pixel-level optically transitioning filter elementsmay be suspended or otherwise operatively disposed above a variety oftypes of detector elements including, but not limited to, infraredmicrobolometer pixel membrane, CCD detector pixel, millimeter wavedetector pixel, CMOS detector pixel, etc. Further, as previouslyindicated herein, a separate optically transitioning filter element maybe monolithically fabricated with a corresponding underlying detectorpixel in a manner as previously described for other embodiments herein,or may be separately fabricated and then attached or otherwise assembledover a corresponding individual detector element.

In the exemplary embodiment of FIG. 24, optically transitioning filterelement 2410 may be composed of one or more optically transitioningmaterials suitable for the filtering requirements of a givenapplication, e.g., selected to filter out undesired wavelengths at agiven selected temperature. Examples of such materials include, but arenot limited to, thermochromic or phase transitioning compositions ofgermanium-antimony-tellurium (GST or Ge_(X)Sb_(Y)Te_(Z)), vanadium oxide(VO, VO₂, V_(n)O_(2n-1) such as V₂O₃), tungsten-doped vanadium oxide(W:VO_(X) such as W:VO₂, and stoichiometric variants), niobium oxide,tantalum oxide, Ti₂O₃, Fe₃O₄, Mo₉O₂₆, etc.). Such opticallytransitioning materials act to selectively transmit radiation dependingon the temperature of the material, e.g., vanadium dioxide (VO₂) remainsan optically transmissive semiconductor to infrared radiation belowabout 68° C. or below and becomes an optically opaque metallic materialto infrared radiation at temperatures greater than or equal to about 68°C. In one particular exemplary embodiment, vanadium oxide having theformula V_(n)O_(2n-1) may be employed as an optically transitioningmaterial for selectively filtering infrared radiation, where n has avalue of from 1 to 5. In any case, it will be understood thatcomposition/s of an optically transitioning material may be chosen thatoptically transition to selectively filter out those particularradiation wavelengths (e.g., including non-infrared radiationwavelengths) that meet the needs of a given application.

Further, as described elsewhere herein, composition of opticallytransitioning materials may be varied (e.g., by doping with othermaterials such as tungsten, aluminum and/or manganese) to tailor theoptical transition (e.g., phase transition) characteristics of thematerial as a function of temperature for different applications. Forexample, in one exemplary embodiment, thermochromic vanadium oxide(e.g., VO₂) may be doped with from about 1% to about 1.9% tungsten tolower the transition temperature, although other amounts and/or types ofdopants may be employed. Thus, an optically transitioning material maybe selected based on its optical transition properties, and/or itsoptical transition properties may be tailored by doping, to configure anoptically transitioning filter that minimizes parasitic effects whilefiltering out any undesired radiation wavelengths at higher temperatureswithout substantially absorbing radiation wavelengths of interest atlower temperatures.

Table 4 illustrates exemplary dimensions for one possible embodiment ofan optically transitioning filtered detector embodiment 2400 using thematerial layer sequence 1600 of FIG. 16, which was previously describedin relation to spectral filter layers, it being understood that avariety of other configurations may be specified as needed to meetdifferent applications. In this regard, it will be understood that inother embodiments, optically transitioning filter element 2410 may beconfigured with different materials, and greater or lesser thicknesses.Likewise, it will be understood that thermal isolation legs 2404 may beconfigured with any dimensions and/or geometry suitable for supportingand contributing to thermal isolation of optically transitioning filterelement 2410.

TABLE 4 Optically Transitioning Filter Layer Structure Layer ThicknessLayer Material (micrometers) 1318 Aluminum 0.25 1606 Vacuum Cavity 2.151608 Silicon Nitride 0.03 1610 Silicon 0.065 1612 Silicon Nitride 0.041614 Titanium 0.025 1616 Silicon Nitride 0.05 1618 Vacuum Cavity 0.81620 Silicon Nitride 0.03 1622 VO₂ 0.2 1624 Silicon Nitride 0.03

FIG. 18B illustrates spectral response in terms of detector absorptanceversus wavelength for the amorphous-silicon two-level detectorembodiment according to the optically transitioning filtered layerstructure of Table 4. It will be understood that the layer structureembodiments of Table 4 is exemplary only, and that other materials,layer configurations and layer thicknesses may be employed as suitableor desired for the given application.

For example, solar radiation consists more predominantly of mid wave IRradiation (from about 3 to about 5 microns) than long wave IR radiation(from about 8 to about 14 microns). Therefore, an infrared detector maybe configured with an optically transitioning material (e.g.,tungsten-doped vanadium oxide (W:VO_(X), and stoichiometric variants)that absorbs mid wave IR radiation, but that is substantiallytransmissive to long wave IR radiation. Therefore, the selectedoptically transitioning material of the filter element passes thedesired radiation waveband (long wave IR) to the underlying detectorelement without substantial heating and while remaining transmissive andbelow its optical transition temperature. However, upon exposure to theundesired waveband (mid wave solar radiation) the selected opticallytransitioning material of the filter element absorbs the undesiredradiation and heats up to above its transition temperature, at whichpoint it becomes opaque and substantially shields the underlyingdetector element from mid wave IR radiation.

Besides the selection of optically transitioning materials, other designfactors that may be varied to tune or otherwise control the filteringcharacteristics of an optically transitioning filter element include,but are not limited to, 1) optically transitioning filter elementthickness to define radiation wavelengths to which the opticallytransitioning filter responds, 2) thermal isolation of the opticallytransitioning filter element from the surrounding environment andunderlying components of the detector element to control the exposure ofthe microbolometer to radiation at a given spectrum, and 3) opticallytransitioning filter element size (surface area) relative to size(surface area) of the underlying microbolometer pixel membrane structure312 or other pixel detector element component/s to control fraction(percentage) of incoming undesired radiation that is absorbed by theunderlying microbolometer. In this regard, optically transitioningfilter element thickness and thermal isolation may be selected to tuneperformance of the optically transitioning filter element such that itbecomes non-transmissive to undesired radiation before an unacceptableamount of the undesired radiation has an opportunity to pass through tothe underlying detector element components.

In the embodiment of FIG. 24, optically transitioning filter elementsupport interconnects 2402 are composed of material that is bothelectrically and thermally insulating (e.g., such as silicon nitride(SiN) or other electrically and thermally insulating material), and maybe configured with similar dimensions as previously described foroptical element support interconnects 320 in relation to FIG. 3. Asshown, optically transitioning filter element support interconnects 2402may be disposed on electrically conductive interconnects 308 (e.g., TiW)as shown to minimize any addition to the pixel thermal mass or change tothe pixel time constant. Similar to other interconnect structuresdescribed herein, optical support interconnects 2402 may be configuredas hollow tubular structures (e.g., cylindrical, square, etc.), althoughsolid interconnects or interconnects having greater or less wallthicknesses may be alternately employed. Configuring opticallytransitioning filter element support interconnects 2402 to be thermallyinsulating helps to thermally isolate optically transitioning filterelement 2410 from the other underlying components of infrared detectorelement 2400, thus allowing optically transitioning filter element 2410to retain heat absorbed from incoming radiation such that thetemperature of optically transitioning filter element 2410 increasesfrom a temperature below its optical transition (e.g., phase transition)temperature to a temperature above its optical transition (e.g., phasetransition) temperature upon exposure to such radiation. In oneexemplary embodiment, optically transitioning filter element supportinterconnects 2402 may be optionally composed of the same material asoptically transitioning filter element 2410 (e.g., vanadium oxide andstoichiometric variants or other suitable optically transitioningmaterial), for example, as a downwardly-protruding extension ofoptically transitioning filter element 2410.

To further increase its thermal isolation, optically transitioningfilter element 2410 may be configured with optional thermal isolationlegs 2404 defined in the optically transitioning material of filterelement 2410 to extend from optically transitioning filter elementsupport interconnects 2402 to support the main body of opticallytransitioning filter element 2410 as shown. The length and crosssectional area of legs 2404 determines the filter's response to IR flux.

FIG. 25 illustrates one exemplary embodiment of an uncooled infrareddetector element 2500 that includes an optically transitioning (e.g.,thermochromic or phase transitioning) polarizer filter element 2510 thatis formed and suspended (e.g., at a distance of about 1 to 2 microns orother suitable distance) by two electrically and thermally insulatingoptically transitioning filter element support interconnects 2402 overand above a microbolometer pixel membrane structure 312 and associatedsilicon semiconductor substrate 216 and optional reflector 318 in amanner similar to that described in relation to FIG. 24. Polarizerfilter element 2510 may be of any patternable optically transitioningmaterial, e.g., of a thickness and type/s described in relation to theembodiment of FIG. 24. As shown, monolithic polarizer filter may bepatterned in a manner similar to that described in relation to theembodiment of FIG. 3 into a linear diffraction grating (e.g., of varyingpossible periods and orientations) that includes elongated gratingelements (wires) 2424 that are separated by elongated spaces 2526. Aswith the embodiment of FIG. 3, dimensions of such a diffraction gratingor other polarizer filter design may vary according to the desiredfiltering characteristics to be imparted by polarizer filter element2510, and grating element width and period may be selected and varied toachieve desired filter effects based on factors such as spectrum orspectra to be filtered, required polarizer efficiency, etc.

In a manner similar to that described in relation to FIG. 4, adjacentuncooled infrared detector elements 2500 of a focal plane array may beprovided with differently-configured optically transitioningpolarization filter elements 2510, e.g., so that adjacent infrareddetector element pixels 2400 absorb different polarization components ofthe scene infrared radiation. For example, FIG. 26 illustrates a topview of one exemplary embodiment of a 2×2 subarray 2600 of four adjacentoptically transitioning polarization filter elements 2510 a-2510 d.Subarray 2600 may form a part of a larger FPA 212, e.g., such as a 30micron pixel 320×240 amorphous silicon microbolometer FPA 212. In FIG.26, 2×2 subarray 2600 includes polarizer filter structures 2510 a-2510 din place overlying corresponding microbolometer pixel membranestructures 312 a-312 d of 2×2 subarray 400 of FIG. 4.

As shown in FIG. 26, the polarizer filter structures include horizontalmonolithic grating structure 2510 c, +45° monolithic grating structure2510 a, −45° monolithic grating structure 2510 d, and verticalmonolithic grating structure 2510 b. In the illustrated embodiment ofFIG. 26, four polarizations (corresponding to four Stokes vectors) areincluded in 2×2 subarray 2600. However, it will be understood that theembodiment of FIG. 26 is exemplary only, and that any other combinationof two or more differently-configured monolithic polarization filterelements 2510 may be employed in the same FPA 212, in a regular orirregular pattern, or in a random arrangement (e.g., in a larger patternsuch as previously described and illustrated in relation to FIG. 6). Aspreviously described in relation to the embodiment of FIG. 7, one ormore microbolometer pixel membrane structures 312 of a focal plane array212 may be left uncovered with no overlying polarizer filter structure2510.

It will be understood that optical elements of one type may be combinedor stacked with one or more other types of optical elements above anunderlying detector element, such as a microbolometer pixel membranestructure 312. For example, optically transitioning filter elements maybe stacked or combined with other types of optical elements such aspolarizing filter elements 322 and/or spectral filter elements 1222. Inthis regard, FIG. 27 illustrates a perspective view of a 2×2 subarray2700 of four adjacent IR detector elements that each include arespective optically transitioning (e.g., thermochromic or phasetransitioning) filter element 2410 a-2410 d suspended by twoelectrically and thermally insulating optically transitioning filterelement support interconnects 2402 (e.g., at a distance of about 1 to 2microns or other suitable distance) over and above a respectivepolarizer filter 322 a-322 d that itself is in turn suspended at adistance of from about 1 micron to about 2 microns over and above arespective microbolometer pixel membrane structure 312 a-312 d by twooptical element support interconnects 320 that each extend down and aresubstantially aligned to and supported by the top of respectivethermally and electrically conductive interconnects 308 of therespective microbolometer pixel structure 312 a-312 d. As shown in FIG.27, optically transitioning filter element support interconnects 2402extend down and are substantially aligned to and supported by the top ofrespective optical element support interconnects 320.

FIG. 28 illustrates a perspective view of another exemplary embodimentof a 2×2 subarray 2800 of four adjacent IR detector elements that eachinclude a respective polarizer filter 322 a-322 d suspended by twooptically transitioning filter element support interconnects 2402 (e.g.,at a distance of about 1 to 2 microns or other suitable distance) overand above a respective optically transitioning (e.g., thermochromic orphase transitioning) filter element 2410 a-2410 d that itself is in turnsuspended at a distance of from about 1 micron to about 2 microns overand above a respective microbolometer pixel membrane structure 312 a-312d by two optically transitioning filter element support interconnects2402 that each extend down and are substantially aligned to andsupported by the top of respective thermally and electrically conductiveinterconnects 308 of the respective microbolometer pixel structure 312a-312 d. As shown in FIG. 27, optically transitioning filter elementsupport interconnects 2402 extend down and are substantially aligned toand supported by the top of respective optical element supportinterconnects 320. It is noted that electrically and thermallyinsulating optically transitioning filter element support interconnects2402 may be configured in this embodiment to support both polarizerfilters 322 a-322 d and optically transitioning filter elements 2410a-2410 d in order to provide thermal isolation for each of opticallytransitioning filter elements 2410 a-2410 d.

It will be understood that the particular configurations of theembodiments of FIGS. 27 and 28 are exemplary only, and that any othersuitable combination of materials, structural configurations,dimensions, optical element types, etc. may be employed. For example,more than one optical element or other types of separate pixel-leveloptical elements (e.g., a separate spectral filter element) may besuspended in the radiation path between an optically transitioningfilter element 2410 and a microbolometer pixel membrane structure 312 orother type of radiation detector circuitry. Likewise, more than onepixel-level optical element or other types of separate optical elements(e.g., a separate spectral filter element) may be suspended over andabove an optically transitioning filter element 2410 that is in turnsuspended over and above a microbolometer pixel membrane structure 312or other type of radiation detector circuitry. Optically transitioningfilter elements are also discussed in concurrently filed patentapplication Ser. No. 12/799,629, now U.S. Pat. No. 8,227,755, entitled“PIXEL-LEVEL OPTICALLY TRANSITIONING FILTER ELEMENTS FOR DETECTORDEVICES” by Fagan III et al., which is incorporated herein by referencein its entirety.

FIG. 29 illustrates a perspective view of an uncooled infrared detectorelement 2900 according to another exemplary embodiment of the disclosedapparatus and methods. In this embodiment, uncooled infrared detectorelement 2900 includes an optically transitioning microbolometer pixelmembrane structure 2912 that includes thin (e.g., from about 1000Angstroms to about 4000 Angstroms thick) thermally sensitive membranematerial of a resistive material like amorphous silicon (a-Si);amorphous silicon geranium (a-SiGe); or Vanadium Oxide (VOx) togetherwith infrared absorbing material that includes one or more opticallytransitioning materials (e.g., thermochromic or phase transitioningcompositions of germanium-antimony-tellurium (GST orGe_(X)Sb_(Y)Te_(Z)), vanadium oxide (VO, VO₂, V_(n)O_(2n-1) such asV₂O₃), tungsten-doped vanadium oxide (W:VO_(X) such as W:VO₂, andstoichiometric variants), niobium oxide, tantalum oxide, Ti₂O₃, Fe₃O₄,Mo₉O₂₆, etc.)) that act to selectively transmit radiation depending ontemperature. As described elsewhere herein, composition of suchmaterials may be varied (e.g., by doping with other materials such astungsten, aluminum and/or manganese) to tailor the optical transition(e.g., phase transition) characteristics of the material as a functionof temperature.

In one exemplary embodiment, the microbolometer pixel membrane structure2912 of infrared detector element 2900 may be suspended about 2 micronsabove silicon semiconductor substrate 216 by relatively long thermalisolation legs 316 that are electrically connected to the ROIC withinsemiconductor substrate 216 by the metal interconnects 308 viaconductive input pads 314, although a microbolometer pixel membranestructure 2912 may be suspended greater than or less than about 2microns above silicon semiconductor substrate 216 in other embodiments.It will be understood that a plurality of infrared detector elements2900 may be employed in a manner similar to infrared detector elements300 to form a focal plane array 212 such as illustrated and described inrelation to FIG. 2.

Still referring to FIG. 29, a metal reflector 318 (e.g., aluminum, etc.)for reflecting IR radiation may be optionally disposed as shown on thesurface of the supporting substrate 216 and ROIC to form a resonantcavity structure to enhance infrared absorptance in the membrane of thesuspended microbolometer pixel structure 2912. In one exemplaryembodiment, pixel geometry may be square-shaped and the pixel area mayrange from about 12 um×12 um to about 100 um×100 um, although pixelshapes other than square and/or pixel sizes smaller than 12 um×12 um orgreater than 100 um×100 um may be employed in other embodiments.Although no material has been removed from the membrane material ofsuspended microbolometer pixel membrane structure 2912, it will beunderstood that a microbolometer pixel structure having a DRCconfiguration (e.g., such as illustrated in FIG. 3) may be similarlyprovided with an optically transitioning (e.g., thermochromic or phasetransitioning) thermally absorbing material.

FIG. 30 illustrates a side cross-sectional view of a material layersequence 3000 for one exemplary embodiment of an uncooled infrareddetector element 2900 having an optically transitioning microbolometerpixel membrane structure 2912 that includes one or more opticallytransitioning materials, such as described and illustrated in relationto FIG. 29. In one embodiment, the layers of FIG. 30 may be formed bydeposition on top of a substrate 216 and ROIC 1602 therein. As shown inFIG. 30, layer sequence 2900 includes ROIC 1602 of substrate 216 withoptional reflective aluminum layer 318 disposed thereon. A cavityseparates substrate 216 and reflective layer 318 from opticallytransitioning microbolometer pixel membrane structure 2912, which itselfincludes silicon nitride layers 1608 and 1612 withthermally-electrically active amorphous silicon layer 1610 therebetween.As shown, an optically transitioning material absorber layer 3014overlays silicon nitride layer 1612, and is overlain itself by siliconnitride layer 1616 to complete microbolometer pixel membrane structure2912. Optically transitioning material layer 3014 may be, for example,at least one thermochromic composition (e.g., Ge_(X)Sb_(Y)Te_(Z),vanadium oxide (V_(n)O_(n-1)), niobium oxide, tantalum oxide, Ti₂O₃,Fe₃O₄, Mo₉O₂₆, etc.) that acts to selectively transmit radiationdepending on temperature. A slit 2920 or other type of discontinuity inabsorber 3014 may be provided as shown to interrupt electricalconductivity across the absorber. Slit 2920 is shown in hidden outlinein FIG. 29 positioned to interrupt electrical conductivity throughabsorber layer 3014 between electrically conductive interconnects 308coupled to opposite ends of microbolometer pixel membrane structure2912. Any other type of discontinuity in absorber 3014 may be providedto prevent electrical conductivity through absorber layer 3014.

It will be understood that the particular layers illustrated in FIG. 30for pixel membrane structure 2912 are exemplary only, and may be variedin number, type and/or thicknesses to obtain the desired infrareddetection and/or optically transitioning characteristics. Moreover, itwill also be understood that a similar microbolometer pixel membranestructure layer sequence 2912 as shown in FIG. 30 may be employed incombination with other detector element configurations of otherembodiments disclosed herein if desired, e.g., such as in the detectorelement 300 of FIG. 3 that includes a monolithic polarizer filter 322suspended over and above the microbolometer pixel membrane structure,the detector element 1200 of FIG. 12 that includes a monolithic spectralfilter 1222 suspended over and above the microbolometer pixel membranestructure 312, and the detector element 1900 of FIG. 20 that includes amicrolens structure 1902 suspended over and above the microbolometerpixel membrane structure.

FIG. 31 illustrates sheet resistance (Rsh) versus temperature at 500amps for various doped compositions of vanadium oxide, specifically for0.005 aluminum doping, for 0.005 manganese doping, and for 0.02manganese doping. FIG. 32 illustrates temperature dependence of parallelRsh infrared light absorption efficiency for the same various dopedcompositions of vanadium oxide that were described in relation to FIG.31, as well as for titanium-aluminum. As may be seen from the plot ofFIG. 32, absorption efficiency of titanium-aluminum remainssubstantially constant with temperature increase from under 50° C. toabove 150° C. However, absorption efficiency of the various opticallytransitioning vanadium oxide compositions decreases by varying amountwith increasing temperatures equal to or greater than about 68° C. Thus,it may be seen that radiation absorption (and therefore temperature)behavior of microbolometer pixel membrane structure 2912 as a functionof absorbed radiation may be controlled by selection of an opticallytransitioning material having the desired radiation absorption versustemperature relationship for the absorber layer of the microbolometerpixel membrane structure.

The simplified illustration of FIG. 33 graphically illustrates howsubstantially all incoming radiation is either absorbed as radiation3302 by microbolometer pixel membrane structure 2912 or is transmittedthrough microbolometer pixel membrane structure 2912 and reflected backup as radiation 3304 to microbolometer pixel membrane structure 2912 atrelatively moderate temperatures substantially below the opticaltransition temperature of the optically transitioning material in themicrobolometer pixel membrane structure 2912, e.g., at temperatures lessthan about 68° C. for an optically transitioning VOx (e.g., VO₂)absorber material 3014 having the absorption efficiency characteristicsillustrated in FIG. 32.

The simplified illustration of FIG. 34 graphically illustrates howradiation absorption is moderated at relatively higher temperatures thatare substantially above the optically transitioning (e.g., thermochromicor phase transitioning) temperature of the material of themicrobolometer pixel membrane structure 2912 such that the opticalcavity 1606 detunes as temperature rises and such that the pixeltemperature is naturally self-limited to provide an increased margin oftemperature safety for the underlying amorphous siliconthermally-electrically active layer 1610. In this regard, only a portionof incoming radiation is absorbed as radiation 3302 by microbolometerpixel membrane structure 2912, while the remainder of the incomingradiation is reflected as radiation 3306 by an optically transitioningabsorber material of microbolometer pixel membrane structure 2912 atrelatively higher temperatures, e.g., at temperatures greater than orequal to about 68° C. for an optically transitioning vanadium oxide(e.g., VO₂) absorber material 3014 having the absorption efficiencycharacteristics illustrated in FIG. 32. As further shown in FIG. 34,less radiation is transmitted through the optically transitioningabsorber material of microbolometer pixel membrane structure 2912 andreflected as radiation 3304 back up to microbolometer pixel membranestructure 2912 at such higher temperatures.

In the exemplary embodiment of FIGS. 29 and 30, optically transitioningmaterial may be incorporated into an absorber layer 3014 toadvantageously provide optically transitioning characteristics to themicrobolometer pixel membrane structure with little or substantially noincrease in thermal mass. However, it will be understood that otherembodiments of optically transitioning microbolometer pixel membranestructures may be implemented by incorporating optically transitioningmaterial in a part/s of a microbolometer pixel membrane structure otherthan the absorber layer. For example, a solid or intermittent opticallytransitioning material layer may be formed on top of a microbolometerpixel membrane structure, e.g., such as formed over a top siliconnitride layer 1616 of a microbolometer pixel membrane structure having aTitanium or Ti—Al absorber layer 1614. Further, it is possible that onlya portion/s of the planar area of a microbolometer pixel membranestructure may be provided with optically transitioning material, i.e.,such that only a portion of incoming radiation is blocked or reflectedby the microbolometer pixel membrane structure with increasingtemperature. For example, an intermittent optically transitioningmaterial layer may be provided that by forming a pattern of localizedoptically transitioning material segments (e.g., as circular dots,square dots, etc.) over a top silicon nitride layer 1616 of amicrobolometer pixel membrane structure having a Titanium or Ti—Alabsorber layer 1614. In this way, regardless of temperature someradiation always passes through a portion's of the microbolometer pixelmembrane structure that is not overlain by optically transitioningmaterial, while radiation is selectively blocked from passing throughthose portions of the microbolometer pixel membrane structure that areoverlain by optically transitioning material.

FIGS. 29-30 and 33-34 illustrate embodiments of an opticallytransitioning radiation detector configured as an opticallytransitioning microbolometer pixel membrane structure. However, it willbe understood that an optically transitioning radiation detector elementmay include any other type of thermal detector element that utilizes anoptically transitioning thermal absorption structure to sense radiationfalling incident thereon by measuring at least one property having avalue that changes with temperature and in which the thermal absorptionstructure is provided with one or more components that are opticallytransitioning. Examples include, but are not limited to, thermaldetector elements that sense radiation by measuring changes in one ormore properties of electrical resistance, electrical capacitance,electrical voltage, electrical current, electromotive force, etc.Specific examples of such thermal detector elements include, but are notlimited to, thermocouple detectors, ferroelectric detectors,microbolometer detectors, etc.

Thus it will be understood with benefit of this disclosure that anoptically transitioning material structure may be associated at thepixel level with an individual radiation detector element (e.g., as anoptically transitioning filter element suspended over and above theindividual radiation detector element, as an integral part of aradiation detector element such as microbolometer pixel membranestructure, etc.) in a manner such that the optically transitioningmaterial structure associated with a given individual radiation detectorelement only affects radiation destined for that given individualradiation detector element and without affecting radiation destined foranother radiation detector element (e.g., of the same focal planearray). Further as illustrated herein, microbolometer pixel membranestructures and optical elements (including optically transitioningfilter elements) may be further characterized as being substantiallyplanar in some embodiments, although this is not always necessary.

It will also be understood that although particular embodiments of thedisclosed optical elements have been illustrated in conjunction withmicrobolometer pixel membrane structures having a diffractive resonantcavity (DRC) configuration, it is also possible that the disclosedoptical elements may be implemented with non-DRC microbolometer pixelmembrane structure configurations.

While the invention may be adaptable to various modifications andalternative forms, specific embodiments have been shown by way ofexample and described herein. However, it should be understood that theinvention is not intended to be limited to the particular formsdisclosed. Rather, the invention is to cover all modifications,equivalents, and alternatives falling within the spirit and scope of theinvention as defined by the appended claims. Moreover, the differentaspects of the disclosed apparatus and methods may be utilized invarious combinations and/or independently. Thus the invention is notlimited to only those combinations shown herein, but rather may includeother combinations.

What is claimed is:
 1. A method of making a focal plane array assembly,comprising forming a plurality of individual thermal detector elementsarranged as an array, each of at least a portion of the plurality ofindividual detector elements comprising an optically transitioningthermal absorption structure and being configured to sense radiationfalling incident thereon by measuring at least one property of thethermal absorption structure that changes value with temperature, thethermal absorption structure being provided with at least one opticallytransitioning material that is an integral part of the thermalabsorption structure itself; and where the method further comprises:selecting a composition of the optically transitioning material totailor the optical transition characteristics of the material as afunction of temperature so that the thermal absorption structure absorbsless of the incident radiation as its temperature increases; and viewinga scene with the focal plane array such that a temperature of theoptically transitioning material of each of the portion of individualthermal detector elements increases while viewing the scene to cause theoptically transitioning material to transition from a semiconductingphase to a metallic phase while viewing the scene.
 2. The method ofclaim 1, further comprising providing a substrate; and where the thermalabsorption structure of each of the portion of individual thermaldetector elements is a microbolometer pixel membrane structure thatprovides a membrane suspended above the substrate that is configured toabsorb radiation incident thereon, the microbolometer pixel membranestructure of each of the microbolometer pixel membrane structures beingdisposed in spaced relationship above the substrate to define a cavitytherebetween with the one or more optically transitioning materialcomponents being formed as an integral part of the membrane itself. 3.The method of claim 1, where each of the portion of individual thermaldetector elements is configured such that the optically transitioningmaterial undergoes a semiconducting-to-metallic phase transition withincreasing temperature; and where the method further comprises selectinga composition of the optically transitioning material to tailor thesemiconducting-to-metallic phase transition characteristics of thematerial as a function of temperature to selectively filter outradiation while viewing a scene by reflecting the radiation from thethermal absorption structure depending on the temperature of thematerial so as to control radiation absorption of the thermal absorptionstructure.
 4. The method of claim 3, further comprising viewing a scenewith the focal plane array such that a temperature of the opticallytransitioning material of each of the portion of individual thermaldetector elements increases while viewing the scene to cause theoptically transitioning material to transition from a semiconductingphase to a metallic phase while viewing the scene to selectively filterout radiation by reflecting the radiation from the thermal absorptionstructure while viewing the scene with the focal plane array.
 5. Themethod of claim 1, where the method further comprises selecting acomposition of the optically transitioning material to exhibit opticaltransition characteristics that filter out undesired radiation byreflecting the undesired radiation from the thermal absorption structureat one or more selected radiation wavelengths and/or at one or moreselected temperatures of the material so as to control radiationabsorption of the thermal absorption structure while the thermaldetector element is viewing a scene that includes the undesiredradiation.
 6. The method of claim 1, further comprising: providing asubstrate; and forming each of the portion of individual thermaldetector elements as an uncooled infrared detector; where the thermalabsorption structure of each of the portion of individual thermaldetector elements is a microbolometer pixel membrane structureconfigured to absorb radiation incident thereon, the microbolometerpixel membrane structure being disposed in spaced relationship above thesubstrate to define a cavity therebetween.
 7. The method of claim 1,further comprising: providing a device wafer and forming the forming theplurality of individual thermal detector elements arranged as an arrayon the device wafer; providing a lid wafer, the lid wafer being at leastpartially transmissive of the incident radiation; and assembling the lidwafer to the device wafer such that the lid wafer allows the incidentradiation to reach the focal plane array assembly through the lid wafer.8. The method of claim 7, further comprising sealingly assembling thelid wafer to the device wafer with a vacuum therebetween to form awafer-level packaged focal plane array assembly.
 9. The method of claim1, further comprising selecting a composition of the opticallytransitioning material to tailor the optical transition characteristicsof the material as a function of temperature so that the thermalabsorption structure reflects more incident radiation as its temperatureincreases due to absorbed incident radiation while viewing a scene so asto limit a maximum temperature of the thermal detector element.
 10. Themethod of claim 1, where the optically transitioning material becomesincreasingly reflective of incident radiation with increasingtemperature such that the thermal absorption structure of each of theportion of individual thermal detector elements absorbs a greater amountof incident radiation at a first temperature than the thermal absorptionstructure absorbs at a second temperature when the second temperature ishigher than the first temperature.
 11. The method of claim 10, where thethermal absorption structure of each of the portion of individualthermal detector elements is a microbolometer pixel membrane structureconfigured to absorb radiation incident thereon, the microbolometerpixel membrane structure being disposed in spaced relationship above thesubstrate to define a cavity therebetween; and where the method furthercomprises: providing a substrate; forming each of the portion ofindividual thermal detector elements with a configuration to receiveincident radiation on a first side of the microbolometer pixel membranestructure that faces away from the cavity; and forming a reflectivelayer on the surface of the substrate of each of the portion ofindividual thermal detector elements between the microbolometer pixelmembrane structure and the substrate such that the opticallytransitioning material allows a greater portion of the total receivedincident radiation to be transmitted through the microbolometer pixelmembrane structure to the reflective layer of the substrate at a firsttemperature than the optically transitioning material allows to betransmitted through the microbolometer pixel membrane structure at asecond temperature that is higher than the first temperature.
 12. Amethod, comprising: viewing a scene with a focal plane array thatincludes a plurality of individual thermal detector elements, at least aportion of the plurality of individual thermal detector elements eachcomprising an optically transitioning thermal absorption structureprovided with at least one optically transitioning material that is anintegral part of the thermal absorption structure itself; and sensingradiation from the scene that falls incident on the thermal detectorelements of the focal plane array by measuring at least one property ofthe thermal absorption structure of each of the thermal detectorelements that changes value with temperature; where the temperature ofthe optically transitioning material of the thermal absorption structureof each of the portion of individual thermal detector elements of thefocal plane array increases due to absorbed incident radiation whileviewing the scene to cause the optically transitioning material of thethermal absorption structure to transition from a semiconducting phaseto a metallic phase while viewing the scene such that the thermalabsorption structure absorbs less of the incident radiation as itstemperature increases while viewing the scene.
 13. The method of claim12, further comprising viewing the scene with the opticallytransitioning material of the thermal absorption structure of each ofthe portion of individual thermal detector elements of the focal planearray in a semiconducting phase and viewing the scene with the opticallytransitioning material of the thermal absorption structure of each ofthe portion of individual thermal detector elements of the focal planearray in a metallic phase; and where the temperature of the opticallytransitioning material of the thermal absorption structure of each ofthe portion of individual thermal detector elements of the focal planearray increases due to absorbed incident radiation while viewing thescene to cause the optically transitioning material to transition from asemiconducting phase to a metallic phase to selectively filter out theincident radiation while viewing the scene with the focal plane array sothat the thermal absorption structure absorbs less of the incidentradiation as its temperature increases.
 14. The method of claim 12,further comprising viewing the scene with the optically transitioningmaterial of the thermal absorption structure of each of the portion ofindividual thermal detector elements of the focal plane array in asemiconducting phase and viewing the scene with the opticallytransitioning material of the thermal absorption structure of each ofthe portion of individual thermal detector elements of the focal planearray in a metallic phase; and where the temperature of the opticallytransitioning material of the thermal absorption structure of each ofthe portion of individual thermal detector elements of the focal planearray increases due to absorbed incident radiation while viewing thescene to cause the optically transitioning material of the thermalabsorption structure to transition such that the thermal absorptionstructure reflects more incident radiation as its temperature increasesdue to absorbed incident radiation while viewing the scene so as tolimit a maximum temperature of the thermal detector element.